Riso-Hudson Enneagram Type Indicator (paid)
This is the full version of the RHETI. While this version is scientifically validated, more extensive and more accurate, the same caution regarding tests still applies.

Enneagram Explorations Enneagram tests
David Fauvre & Katherine Chernick Fauvre have done extensive work in the areas of the Instinctual Variants and Tritype®. Here you can their tests, some are free and some are paid.

Riso-Hudson type compatibility chart
While there are no specific type combinations that are likely to be more successful than others, some conclusions can be drawn about the strengths and potential pitfalls of each combination. These detailed and enlightening descriptions can help shed some light on what to expect.

Riso-Hudson Levels of Development
The Levels of Development are a unique contribution to the Enneagram work that Don Riso made in the early part of his career. They remain an incredibly useful tool for describing the wide variance of expression within each of the nine types as well as a good way of knowing how we ourselves are doing with our own self work.

Riso-Hudson Misidentifications
Whether trying to determine our own type or that of someone close to us, this is a great tool for determining differences between types that can look alike in some ways.

Sounds True
Sounds True is one of the best sources for high-quality books and multimedia programs to support personal and spiritual growth. Click this link to sign up for their Weekly Wisdom e-mails and in exchange, you get two free downloads. Their free podcasts are also highly recommended.

Books:

Personality Types: Using the Enneagram for Self-DiscoveryIf you are new to the Enneagram, this is where you should start. It is an exhaustive breakdown of the system and each personality type and includes descriptions of the wings and each type at all of the nine Levels of Development.

Enneagram Transformations
This book was written with twelve-step programs in mind, but has much to offer even to those outside of these programs. It gets to the heart of the cognitive errors each type makes and if used well can go a long way in supporting our growth.